Work continues on the nature and physiological role of class I major histocompatibility antigens in the hamster. A series of studies on virally-transformed hamster fibroblast cell lines has revealed a heterogeneous group of virally-directed or transformation-related changes in expression of class I MHC antigens but as yet no evidence exists that these changes are causally related to differences in tumorigenicity among these cell lines. A set of clones encoding class I antigens derived from a hamster genomic library have been evaluated and 18 unique clones have been identified. Which of these, if any, represents the major cell-surface transplantation antigens has been understudy using cDNA cloning techniques, but the answer has yet to be determined. Another example of anti-allotype induced "latent allotypes" in the rabbit group alpha allotype system has been confirmed by sequence from another independent laboratory. Studies of murine lambda light chain genes in the wild derived mouse strain SPE have resulted in characterization of genes analogous to Clambda2, Clambda4, Clambda5 and Vlambda 2 in laboratory mice. The SPE Clambda2 gene apparently encodes a functional product whereas Clambda4 is obviously a pseudogene by virtue of large deletions in the coding sequence. Only a single Vlambda gene with homology to Vlambda1 or Vlambda2 of laboratory mice was found for SPE. The analog of Clambda5 of C57 mice was studied in SPE and found to be highly conserved. In addition, Clambda5 sequences were seen in all wild mouse strains tested. The gene appeared to be present as a single copy even in strains that have greatly expanded Clambda1 or Clambda2 gene families.